Sensors based on nanosstructured composite films

ABSTRACT

An electrically reactive composite article comprising a random or regular array of microstructures partially encapsulated within an encapsulating layer, microstructures each comprising a whisker-like structure, optionally having a conformal coating enveloping the whisker-like structure is described. The composite article is useful as an electrically conducting component of a circuit, antenna, microelectrode, reactive heater, and multimode sensor to detect the presence of vapors, gases, or liquid analystes.

BACKGROUND OF THE PRESENT INVENTION

1. Field of the Invention

This invention relates to a composite article comprising randomly orregularly arrayed oriented microstructures partially encapsulated withina layer, in particular to the method of making the same and to the useof the composite article as an electrically conducting polymer, thinfilm resonant circuit, antenna, microelectrode or resistive heater, andas a multimode sensor to detect the presence of vapors, gases, or liquidanalytes.

2. Background Art

Composite articles containing or exhibiting a layered structure havebeen prepared by many different types of chemical and physicaldeposition processes.

For example, U.S. Pat. No. 4,812,352 discloses an article comprising asubstrate having a microlayer (microstructured-layer) that comprisesuniformly oriented, crystalline, solid, organic microstructures, severaltens of nanometers in cross-section and a method of making the same.Further, '352 teaches optionally conformal coating the microlayer andencapsulating the conformal-coated microlayer.

Dirks et al. in "Columnar Microstructures in Vapor-Deposited ThinFilms,"Thin Solid films, vol. 47, (1977), pgs 219-33 review severalmethods known in the art that can yield columnar microstructures,however, as Dirks et al. point out the structures are not a desirable ora sought-after outcome of vapor-deposition.

U.S. Pat. No. 3,969,545 describes a vacuum deposition technique that canproduce organic or inorganic microstructures.

Floro et al. in "Ion-Bombardment-Induced Whisker Formation of Graphite,"J. Vac. Sci. Technol. A. vol. 1, no. 3, July/September (1983) pgs1398-1402 describe graphite whisker-like structures produced by anion-bombardment process.

Flexible conducting media known in the art, typically having a layeredstructure, exist in a variety of distinct formats. For example, U.S.Pat. No. 4,674,320 discloses a conducting powder-like material, such ascarbon, dispersed throughout a polymeric binder at concentrationssufficient to enable conduction by charge transfer from particle toparticle. Such an arrangement results in an isotropicly conductingsheet, that is, resistivity perpendicular to the plane of the sheet isthe same as the in-plane resistivity.

Bartlett et al., Sensors and Actuators, vol. 20, pg 287, 1989, disclosea conductive polymer film made by electrochemical polymerization.Resistivities of these polymer films are three-dimensionally isotropicand tend to be relatively high.

Other examples known in the art teach an article comprising a conductinglayer applied to a flexible polymer sheet by vacuum coating processes,electrochemical or electroless plating processes, printing, particleembedding and the like. However, in these cases, the conductive coating,for example, a solid metallic layer, will have a low resistivity and isnot easily controllable. Additionally, since the conductive layer is onthe surface of a polymer substrate, adhesion of the conductive layer tothe polymer substrate is often a problem. The adhesion problem isparticularly apparent when the conducting layer is carrying current. Ifa very thin or discontinuous conductive layer is applied to the polymersubstrate to increase the surface resistivity, the power carryingcapability of the conductive layer tends to be compromised and theproblem of adhesion tends to be exacerbated.

Electrical properties are useful as sensors, however, most prior art gasand vapor sensors are based on many of the prior art layered structures.The sensor media can be thin or thick film devices utilizing eithersurface acoustic wave (SAW) technology or chemiresistors incorporatingsolid electrolytes, polymers with bulk gas sensitivity, metal orsemiconductor (inorganic or organic) thin films, or homogeneousdispersions of conducting particles in insulating matrices.

Generally, sensors based on SAW technology are costly to manufacture andtend to be used only for reversible sensing. They are generally not usedfor nonreversible sensors, such as dosimetry monitoring, see Snow etal., "Synthesis and Evaluation of Hexafluorodimethyl carbinolFunctionalized Polymers as SAW Microsensor Coatings," Polymer Reprints,30(2), 213 (1989); Katritzky et al., "The Development of New MicrosensorCoatings and a Short Survey of Microsensor Technology," AnalyticalChemistry 21(2), 83 (1989).

On the other hand, chemiresistor based sensors tend to be reversible ornonreversible depending on the chemical and physical composition of thesensing medium, see Katritzky et al., "New Sensor Coatings for theDetection of Atmospheric Contaminants and Water," Review of HeteroatomChemistry, 3, 160 (1990). Generally, the prior art sensing media exhibitisotropic or homogeneous gas sensing properties. Media having anisotropic sensing property display the same resistivity in alldirections of the media. Such media are typically capable of only asingle mode of detection. In contrast, media having an anisotropicimpedance sensing property display different in-plane and out-of-planegas sensing impedances. Thus, anisotropic media permit multi-modeoperation.

Generally, conduction through chemiresistor devices occurs betweenconducting particles dispersed throughout the media. For example, U.SPat. No. 4,674,320 teaches a chemiresistive gas sensor comprising alayer of organic semiconductor disposed between two electrodes, whereindispersed within the layer of organic semiconductor is a highconductivity material in the form of very small particles, or islands.Adsorption of a gaseous contaminant onto the layer of organicsemiconductor or modulates the tunneling current.

U.S. Pat. No. 4,631,952 discloses an apparatus and a method for sensingorganic liquids, vapors, and gases that includes a resistivity sensormeans comprising an admixture of conductive particles and a materialcapable of swelling in the presence of the liquid, gas, or vaporcontaminant.

Ruschau et al., "0-3 Ceramic/Polymer Composite Chemical Sensors,"Sensors and Actuators, vol. 20, pgs 269-75, (1989) discloses a compositearticle consisting of carbon black and vanadium oxide conductive fillersin polyethylene, a polyurethane, and polyvinyl alcohol for use aschemical sensors. The polymer matrices swell reversibly in the presenceof liquid and gaseous solvents, disrupting the conductive pathway andproportionally increasing the resistance.

U.S. Pat. No. 4,224,595 discloses an adsorbing type sensor havingelectrically conductive particles embedded in a surface, forming anelectrically conductive path through the sensor.

U.S. Pat. No. 4,313,338 discloses a gas sensing device comprising a gassensing element comprising a gas-sensitive resistive film formed of anaggregate of ultrafine particles of a suitable material deposited on thesurface of a substrate of an electrical insulator formed withelectrodes.

U.S. Pat. No. 3,820,958 discloses an apparatus and a method fordetermining the presence of hydrogen sulfide in a gas mixture. Silver isdeposited on a thin dielectric film. Electrical resistance across thefilm before and after exposure of the film to hydrogen sulfidecontaining gas mixture is utilized to determine the amount of hydrogensulfide present.

U.S. Pat. No. 4,906,440 discloses a sensor for a gas detector comprisinga metallic/metallic oxide gas sensitive discontinuous film. The gaschanges the conductivity of the film and causes the RC network to react.

U.S. Pat. No. 3,045,198 discloses a detection device comprising anelectrical element sensitive to exposure to liquids, vapors or gases.The detection element includes a broad and long base having anelectrically non-conductive, relatively resilient surface on which isanchored a stratum of exposed electrically conductive discrete adsorbentparticles.

Sadaoka et al., Effects of Morphology on NO₂ Detection in Air at RoomTemperature with Phthalocyanine Thin Films," J. of Mat'l Sci. 25, 5257(1990) disclose that crystal size in films is affected by the nature ofthe substrate, ambient atmosphere, and annealing time. The variations ofthe crystals can effect the detection of NO₂ in air.

SUMMARY OF THE PRESENT INVENTION

Briefly, this invention provides a composite article with anelectrically conductive surface comprising a layer having a dense arrayof discrete, oriented microstructures partially encapsulated andoptionally having a conformal coating wherein one end of themicrostructures is exposed and coincident with the conductive surface.The conformal coating, preferably is a conducting material. Theencapsulant, is preferably a dielectric. Advantageously, the anisotropicstructure of the composite article provides anisotropic impedance, thatis, the impedance parallel to the surface plane of the composite articleis resistive, while the impedance perpendicular to the surface plane ofthe article is predominantly capacitive.

In another aspect, a resonant circuit is described wherein the compositearticle provides the resistive (R) and capacitive (C) component of thecircuit. Advantageously, the resonant circuit can be constructed as alow-pass filter, a high-pass filter, a band-pass filter and the like.Furthermore, the composite article can be fabricated such that theconducting layer is formed in patterns suitable for building electroniccircuits. This is achieved, by depositing the crystallinemicrostructures in patterns, or conformally coating the microstructuresthrough a mask, or by encapsulating the coated microstructures through amask, or by any combination of the above.

In yet another aspect of the present invention, a multimode sensor isdescribed. The unique construction of the composite article enablesselection of the conformal coating and the encapsulant for theirresponses to a particular analyte molecule of interest. The effect ofgas/vapor/liquid molecules on the multimode sensor is detected bymonitoring the changes in the composite article's electrical properties,that is the resistance and the capacitance.

In this application:

"whisker-like structure" refers to individual repeating units such as,for example, material structures, whiskers, rods, cones, cylinders,laths, pyramids and other regular or irregular geometric shapedstructures;

"microstructure" refers to the whisker-like structure that has beenconformally coated;

"microstructured-layer" refers to a layer formed by all themicrostructures taken together;

"conformal-coated" means a material is deposited onto the sides and anend of each whisker-like structure element to envelope the element suchthat the deposited material conforms to the shape of the whisker-likestructure element;

"uniformly oriented" means the microstructures are approximatelyperpendicular to the surface of the substrate;

"solidified" means the encapsulant undergoes a change in state,typically from a liquid or liquid-like phase to a more rigid, solid, orsolid-like phase, such as may occur as a result of drying, chemicalsetting, cooling, freezing, gelling, polymerization, etc.;

"continuous" means coverage of a surface without interruption of thecoating;

"discontinuous" means coverage of a surface wherein there is periodic ornon-periodic interruption of the coating;

"uniform" with respect to size, means that the major dimension of thecross-section of the individual microstructures varies no more thanabout ±25% from the mean value of the major dimension and the minordimension of the cross-section of the individual microstructures variesno more than about ±25% from the mean value of the minor dimension;

"areal number density" means the number of microstructures per unitarea;

"gas" means a state of matter existing in the gaseous state at standardtemperature and pressure, but can be liquified by pressure; and

"vapor" means an air dispersion of molecules of a substance that isliquid or solid in its normal state, that is at standard temperature andpressure, sometimes called fumes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a perspective view of an article with ananostructured composite surface being delaminated from a substrateaccording to the present invention with a cut away portion showing thecomposite whisker-like structures.

FIGS. 2 (a) and (b) illustrate a three terminal AC electric circuitconfiguration using metal foil tape as one contact to the invention andits simplified representative RC schematics.

FIGS. 3 (a) and (b) illustrate two strips of nanostructured compositemedia, as shown in FIG. 1, adjacently positioned to form a four terminalnetwork configuration and a simplified equivalent circuit.

FIGS. 4 (a) and (b) illustrate an alternative "series" configurationwith band-pass characteristics and a simplified equivalent circuit.

FIG. 5 is the graphic representation of the low-pass AC filter transferfunction measured for Examples 1 and 2.

FIG. 6 is the graphic representation of the three terminal networklow-pass frequency response functions for Examples 3 to 10.

FIG. 7 is the graphic representation of the three terminal networkhigh-pass frequency response function for Examples 3 to 10.

FIG. 8 is the graphic representation of the temperature rise of thenanostructured surface versus electrical power dissipated by theconducting layer of the composite media of Examples 20-23 of the presentinvention when the composite layer is heat-sinked (a to c), compared toa Co film sputtered onto polyimide.

FIGS. 9 (a) and (b) is the graphic representation of the resistancechange of type B sample of Examples 24 and 31.

FIG. 10 is the graphic representation of the sensitivity versus exposuretime for type C samples of Example 30.

FIG. 11 is the graphic representation of the resistance change versustime for type E samples of Example 32.

FIG. 12 is the graphic representation of the sensitivity versus time atseveral temperatures and vapor pressure fractions for type F samples ofExample 34.

FIG. 13 is the graphic representation of the sensitivity versus time atseveral temperatures and vapor pressure fractions for type G samples ofExample 35.

FIG. 14 is the graphic representation of the sensitivity versus time forsaturated water vapor at several temperatures for type E samples ofExample 36.

FIG. 15 is the graphic representation of the capacitance change versustime for B, E, L, and M type samples of Example 40.

FIG. 16 is an Arrhenius plot for the rate kinetics for water vaporoxidation of Cu coated whisker composite media of type E.

FIG. 17 is a graphic representation of the linear relationship of thesensor resistance changes versus the log of the initial resistivity,wherein the slope is ostensibly proportional to the analyteconcentration.

FIG. 18 is a solid state diffusion model representation of sensitivityversus time data from FIG. 12.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention discloses a composite article having anelectrically conducting surface, the process for making such a film withvariable surface resistivity, and use of the invention as a flexibleelectric circuit element having both capacitance and resistanceproperties are described. Specific examples are given demonstrating thesuitability of the media for use directly as passive RC filter networkswith significant power dissipation potential. Additional examplesdemonstrate the suitability of the media for use as gas, vapor andliquid analyte sensors that derive sensing properties from theproperties of the nanostructured composite film surface. The sensorfunctions uniquely in two distinct ways, first in terms of the dualmechanisms by which vapor/gas/liquid molecules affect sensor properties,and secondly with respect to the independent resistance and capacitanceimpedance properties that can be measured as a function of exposure tothe vapor, gas, or liquid.

Referring to FIG. 1, composite article 20 comprises encapsulant layer12, for example, a polymer that has encapsulated in layer 12 arrayedmicrostructures 16, which may also be composites preferably initiallyoriented normal to the substrate 11. Each microstructure 16 compriseswhisker-like structure 14 and optionally, conformal coating 13enveloping whisker-like structure 14. The chemical composition ofmicrostructures 16 is determined by the starting material deposited onsubstrate 11 to form the whisker-like structures 14 and the conformalcoating 13 subsequently applied to the whisker-like structures 14.Microstructures 16 may be randomly or regularly arrayed in encapsulatinglayer 12.

As shown in FIG. 1, composite article 20 is partially delaminated fromsubstrate 11 and delamination of composite article 20 is occurring atinterface 15. Delamination of the composite article 20 from substrate 11takes microstructures 16 along, embedded precisely in the surface ofencapsulant layer 12 and exposes one cross-sectional end of eachmicrostructure 16, wherein a surface of the- encapsulating material ofencapsulant layer 12 and the exposed cross-sectional ends ofmicrostructures 16 are coincident on a common side. The topography ofthe delaminated surface, or exposed surface of the composite article 20is the inverse of the topography of the surface of substrate 11 fromwhich it is delaminated. Furthermore, the exposed surface of thecomposite article 20 is electrically reactive, that is, exhibits surfaceelectronic phenomena, such as resistance and capacitance. If the surfaceof substrate 11 is perfectly smooth, the exposed cross-sectional ends ofmicrostructures 16 and the delaminated surface of the encapsulatinglayer will be on a common plane.

The unique fracture and adhesion properties of whisker-like structures14 at substrate interface 15 allow whisker-like structures 14 towithstand the coating and encapsulating processes, yet be easily andcleanly delaminated from substrate 11.

The thickness of conformal coating 13 applied to whisker-like structures14, and intrinsic resistivity of said conformal coating 13, are theprimary parameters controlling the surface electronic conductivity ofcomposite article 20.

It should be noted that composite article 20 can have the conductingarea formed into patterns suitable for building electronic circuits byseveral means. For example, starting material, for example, perylenepigment, can be deposited through a mask, or the conducting conformalcoating can be applied to the whisker-like structures through a mask, orthe encapsulant can be photolithographically applied to encapsulate thecoated whisker-like structures image-wise. The small volume andflexibility of the medium of the present invention allows it to be usedin a wide variety of resonant circuit constructions. Additionally, theexposed surface of the composite article, that is the reactive surface,can be coated in a patterned manner with an insulator or otherdielectrics.

Materials useful as a substrate for the present invention include thosewhich maintain their integrity at the temperatures and pressures imposedupon them during any deposition and annealing steps of subsequentmaterials applied to the substrate. The substrate may be flexible orrigid, planar or non-planar, convex, concave, aspheric or anycombination thereof.

Preferred substrate materials include organic or inorganic materials,such as, polymers, metals, ceramics, glasses, semiconductors. Preferredorganic substrates include polyimide film, commercially available underthe trade designation KAPTON™ from DuPont Corp., Wilmington, Del.Additional examples of substrate materials appropriate for the presentinvention can be found in U.S. Pat. No. 4,812,352 and is incorporatedherein by reference.

Starting materials useful in preparing the whisker-like structuresinclude organic and inorganic compounds. The whisker-like structures areessentially a non-reactive or passive matrix for the subsequentconformal coating and encapsulating material. In addition to startingmaterials that produce whisker-like structures, several techniques ormethods are useful for producing the whisker-like configuration of theparticles.

For example, methods for making organic microstructured layers aredisclosed in J Sci. Technol. A, vol. 5, no. 4, July/August (1987), pgs1914-16; J. Sci. Technolol. A, vol. 6, no. 3, May/June (1988), pgs1907-11; Thin Solid Films, vol. 186, (1990), pgs 327-47; U.S. Pat. No.3,969,545; Rapid Quenched Metals, (Proc. of the Fifth Int'l Conf. onRapidly Quenched Metals), Wurzburg, Germany, Sept. 3-7 (1984); S. Steebet al. Eds. Elsevier Science Publishers B.V., New York (1985), pgs1117-24; U.S. Pat. No. 4,568,598; Photo. Sci. and Eng., vol. 24, no. 4,July/August, (1980), pgs 211-16; and U.S. Pat. No. 4,340,276, thedisclosures of which are incorporated herein by reference.

Methods for making inorganic-, metallic-, or semiconductor-basedmicrostructured-layers or whisker-like structures are disclosed in U.S.Pat. No. 4,969,545; J. Vac. Sci. Tech. A, vol. 1, no. 3, July/Sept.(1983), pgs 1398-1402; U.S. Pat. No. 4,252,864; U.S. Pat. No. 4,396,643;U.S. Pat. No. 4,148,294; U.S. Pat. No. 4,155,781; and U.S. Pat No.4,209,008, the disclosures of which are incorporated herein byreference.

The organic compounds include planar molecules comprising chains orrings over which π-electron density is extensively delocalized. Theseorganic materials generally crystallize in a herringbone configuration.Preferred organic materials can be broadly classified as polynucleararomatic hydrocarbons and heterocyclic aromatic compounds. Polynucleararomatic hydrocarbons are described in Morrison and Boyd, OrganicChemistry 3rd ed., Allyn and Bacon, Inc. (Boston, 1974), Chap. 30.Heterocyclic aromatic compounds are described in Chap. 31 of the samereference.

Preferred polynuclear aromatic hydrocarbons include, for example,naphthalenes, phenanthrenes, perylenes, anthracenes, coronenes, andpyrenes. A preferred polynuclear aromatic hydrocarbon isN,N'-di(3,5-xylyl)perylene-3,4:9,10 bis(dicarboximide), commerciallyavailable under the trade designation of C. I. Pigment Red 149 (AmericanHoechst Corp., Sommerset, N.J.) [hereinafter referred to as perylenered].

Preferred heterocyclic aromatic compounds include, for example,phthalocyanines, porphyrins, carbazoles, purines, and pterins. Morepreferred heterocyclic aromatic compounds include, for example,porphyrin , and phthalocyanine, and their metal complexes, for examplecopper phthalocyanine. Such a compound is available, from Eastman Kodak,Rochester, N.Y.

The organic material for whisker-like structures may be coated onto asubstrate using well-known techniques in the art for applying a layer ofan organic material onto a substrate including but not limited to vacuumevaporation, sputter coating, chemical vapor deposition, spray coating,Langmuir-Blodgett, or blade coating. Preferably, the organic layer isapplied by physical vacuum vapor deposition (i.e., sublimation of theorganic material under an applied vacuum). The preferred temperature ofthe substrate during deposition is dependent on the organic materialselected. For perylene red, a substrate temperature near roomtemperature (i.e., about 25° C.) is satisfactory.

In the preferred method for generating organic whisker-like structures,the thickness of the organic layer deposited will determine the majordimension of the microstructures which form during an annealing step.Whisker-like structures 14 are grown on a substrate 11 with thecharacteristics and process described in U.S. patent application Ser.No. 07/271,930, now U.S. Pat. No. 5,035,961 , filed Nov. 14, 1988 andincorporated herein by reference. The process for obtaining whisker-likestructures 14 is also described in Example 1 hereinbelow. Preferably,when the organic material is perylene red the thickness of the layer,prior to annealing is in the range from about 0.05 to about 0.25micrometer, more preferably in the range of 0.05 to 0.15 micrometer. Theorganic materials are annealed and produce a whisker-like structure.Preferably, the whisker-like structures are monocrystalline orpolycrystalline rather than amorphous. The properties, both chemical andphysical, of the layer of whisker-like structures are anisotropic due tothe crystalline nature and uniform orientation of the microstructures.

Typically, the orientation of the whisker-like structures is uniformlyrelated to the substrate surface. The structures are preferably orientednormal to the substrate surface, that is, perpendicular to the substratesurface. Preferably, the major axes of the whisker-like structures areparallel to one another. The whisker-like structures are typicallyuniform in size and shape, and have uniform cross-sectional dimensionsalong their major axes. The preferred length of each structure is in therange of 0.1 to 2.5 micrometers, more preferably in the range of 0.5 to1.5 micrometers. The diameter of each structure is preferably less than0.1 micrometer.

Preferably, whisker-like structures 14, shown in FIG. 1, aresubstantially uniaxially oriented. Microstructures, 16, are asubmicrometer in width and a few micrometers in length, and arecomposites comprising an organic pigment core whisker conformally coatedwith a conducting material.

The whisker-like structures preferably have a high aspect ratio, (i.e.,a length to diameter ratio in the range from about 3:1 to about 100:1).The major dimension of each whisker-like structure is directlyproportional to the thickness of the initially deposited organic layer.The areal number densities of the conformally coated microstructures 16are preferably in the range of 40 to 50 per square micrometers.

The conformal coating material will generally strengthen themicrostructures comprising the microstructured-layer. Preferably, theconformal coating material has electrically conductive properties and isselected from the group consisting of an organic material, such aselectrical conducting organic materials, for example see "the OrganicSolid State" Cowen et al., Chem & Eng. News, July 21 (1986) pgs 28-45, ametallic material, or a semiconductor inorganic material, such assilicon or gallium arsenide. More preferably, the conformal coatingmaterial is a metal or metal alloy. Preferably, the metallic conformalcoating material is selected from the group consisting of aluminum,cobalt, nickel chromium, cobalt chromium, copper, platinum, silver,gold, iron, and nickel.

Preferably, the organic conformal coating material is selected from thegroup consisting of hetrocyclic polynuclear aromatics. The preferredinorganic conformal coating material is a semiconductor.

Preferably, the wall thickness of the conformal coating surrounding thewhisker-like structure is in the from about 0.5 nanometers to about 30nanometers.

The conformal coating may be deposited onto the microstructured-layerusing conventional techniques, including, for example, those describedin U.S. patent application Ser. No. 07/271,930, now U.S. Pat. No.5,035,961, supra. Preferably, the conformal coating is deposited by amethod that avoids the disturbance of the microstructured-layer bymechanical or mechanical-like forces. More preferably, the conformalcoating is deposited by vacuum deposition methods, such as, vacuumsublimation, sputtering, vapor transport, and chemical vapor deposition.

Preferably, the encapsulating material is such that it can be applied tothe exposed surface of the conformal-coated microstructured-layer in aliquid or liquid-like state, which can be solidified. The encapsulatingmaterial may be in a vapor or vapor-like state that can be applied tothe exposed surface of the conformal-coated microstructured-layer.Alternatively, the encapsulating material is a solid or solid-likematerial, preferably powder or powder-like, which can be applied to theexposed surface of the conformal-coated microstructured-layer,transformed (e.g., by heating) to a liquid or liquid-like state (withoutadversely affecting the conformal-coated microstructured-layercomposite), and then resolidified.

More preferably, the encapsulating material is an organic or inorganicmaterial. The encapsulating material may exhibit sensitivity to gas orvapor contaminants to be detected. Additionally, it is preferable,although not required, that the encapsulant be permeable to gas or vaporcontaminants.

Preferred organic encapsulating materials are molecular solids heldtogether by van der Waals' forces, such as organic pigments, includingperylene red, phthalocyanine and porphyrins and thermoplastic polymersand co-polymers and include, for example, polymers derived from olefinsand other vinyl monomers, condensation polymers, such as polyesters,polyimides, polyamides, polyethers, polyurethanes, polyureas, andnatural polymers and their derivatives such as, cellulose, cellulosenitrate, gelation, proteins, and rubber. Inorganic encapsulatingmaterials that would be suitable, include for example, gels, sols, orsemiconductor, or metal oxides applied by, for example, vacuumprocesses.

Preferably, the thickness of the coated encapsulating material is in therange from about 1 micrometer to about 100 micrometers, and morepreferably in the range from about 6 micrometers to about 50micrometers.

The encapsulating material may be applied to the conformal-coatedmicrostructured-layer by means appropriate for the particularencapsulating material. For example, an encapsulating material in aliquid or liquid-like state may be applied to the exposed surface of theconformal-coated microstructured-layer by dip coating, vaporcondensation, spray coating, roll coating, knife coating, or bladecoating or any other coating method known to those skilled in the art.An encapsulating material may be applied in a vapor or vapor-like stateby using conventional vapor deposition techniques including, forexample, vacuum vapor deposition, chemical vapor deposition, or plasmavapor deposition.

An encapsulating material which is solid or solid-like may be applied tothe exposed surface of the conformal-coated microstructured-layerliquified by applying a sufficient amount of energy, for example, byconduction or radiation heating to transform the solid or solid-likematerial to a liquid or liquid-like material, and then solidifying theliquid or liquid-like material.

The applied encapsulating material may be solidified by meansappropriate to the particular material used. Such solidification meansinclude, for example, curing or polymerizing techniques known in theart, including, for example, radiation, free radical, anionic, cationic,step growth process, or combinations thereof. Other solidification meansinclude, for example, freezing and gelling.

After the polymer is cured, the resulting composite article 20comprising a conformal-coated microstructured-layer and an encapsulatinglayer 12 is delaminated from the substrate 11 at the original substrateinterface 15 (see FIG. 1) by mechanical means such as, for example,pulling the composite layer from the substrate, pulling the substratefrom the composite layer, or both. In some instances, the compositelayer may self-delaminate during solidification of the encapsulatingmaterial.

Capacitive properties of the composite article are determined by thedielectric constants of the encapsulating material, film thickness andplanar area used. Intimate contact of the conductive particles with thesurrounding encapsulant permits the full dielectric response of theencapsulant to be realized with only physical contact of a circuit leadto the conducting side of the composite, that is, without evaporation orsputter coating of a metal overlayer on the polymer surface as isusually necessary to bring a conductor into full electrical contact witha dielectric surface.

It is also a unique property of this medium's structural anisotropy thatthe complex impedance is anisotropic. That is, the impedance parallel tothe surface of the composite film is predominantly resistive, while theimpedance in the direction perpendicular to the surface is predominantlycapacitive, being determined by the very large reactance of the muchthicker encapsulant layer.

FIGS. 2 (a) and (b) illustrate a configuration utilizing the passiveresistance (R) and capacitance (C) properties of thin flexible strips ofthe composite article of this invention. In all cases the resistance andcapacitance character is spatially distributed over the entire area ofthe composite article. Referring to FIG. 2(a), a conductive metal foiltape is applied to the side of the encapsulating polymer opposite theconducting nanostructured side. Electrical contact can then be made atthe three points x, y, and z. This is equivalent in a first orderapproximation to the three terminal network, as shown in FIG. 2(b).Depending on which pairs of terminals are used as input and output foran alternating current (AC) voltage signal, the composite strip canfunction as a low-pass or high-pass filter circuit. For example,applying the input signal across terminals x and z (or y and z) andtaking the output across terminals y and z (or x and z) is equivalent toa low-pass filter. On the other hand, applying the input acrossterminals z and y (or z and x) and the output across terminals x and y(or y and x) produces a high-pass filter. Applying the output and inputsignals to the third combination of terminal pairs, for example, aninput signal applied across terminals x and y (or y and x) and outputmeasured across terminals z and y (or z and x), gives a simplecapacitively coupled voltage divider. The metal foil tape need not beapplied in a single piece and thus could produce multiple terminals. Asused herein, "low-pass filter" means a filter network that passes allfrequencies below a specified frequency with little or no loss. The term"high-pass filter" means a wave filter having a single transmission bandextending from some critical frequency up to infinite frequency. Theterm "voltage divider" means a resistor or reactor connected across avoltage and tapped to make a fixed or variable fraction of the appliedvoltage available.

FIGS. 3 (a) and (b) illustrate a configuration that forms a fourterminal network with two pieces of the nanostructured composite filmarranged in "parallel" such that the conductive sides of the compositefilm are facing outward and the corresponding equivalent electriccircuit. Electrical contact is made at the ends of each side,illustrated as points w, x, y, and z as shown. This can similarly beutilized to have different filter characteristics depending on thevarious combinations of terminals used for input and output, or to formvarious two and three terminal networks.

FIGS. 4 (a) and (b) illustrate a configuration to form a four terminalnetwork with two pieces of the composite film arranged in "series" and asimplified equivalent electrical circuit. This arrangement isapproximately equivalent to a band pass filter. Electrical contact ismade at the ends of each side at points w, x, y, and z as shown. Theterm "band pass filter" means a wave filter with a single transmissionband, wherein the filter attenuates frequencies on either side of thisband. The metal foil tape 22 can be replaced with other compositestrips.

Referring again to FIG. 1, as a sensor, it is found that the in-planesurface resistivity of the nanostructured side of the composite article20, the impedance to current flow in the plane of the whisker-likestructures 14, is a simple yet sensitive probe of gas, vapor, or liquidanalyte effects. The electrical conductance mechanism may involve bothelectron "percolation" from point-to-point where adjacent whiskerstouch, and tunneling through or charge injection into the thinintermediate encapsulating material 12 interstitially located betweenthe conductive conformal coated microstructures 16. Therefore, if theconductivity of this conformal coating 13 applied to the whisker-likestructures 14, or the relative separation of the microstructures 16, orthe charge transport properties of the intermediate encapsulatingmaterial 12 are affected by the analyte, the surface impedance of thecomposite article 20 is altered. The initial surface resistivity iseasily varied over a wide range by controlling the thickness of theconductive conformal coating 13 applied to the whisker-like structure 14prior to encapsulation.

Sensor medium is produced in a convenient flexible polymer form whichmay be cut into arbitrary sizes and shapes. Electrical connections aresimply made by contact with the conducting, chemically active surface.

Referring to FIG. 1, again the physical structure of the compositearticle 20, utilized as a gas, liquid or vapor sensor, comprises apolymer film 12, optionally, sensitive to the vapor or gas of interest,having encapsulated in its surface a dense, random array of discretewhisker-like structures 14. The whisker-like structures 14 are typicallyabout one to a few micrometers in length and submicrometer in width.Microstructures 16 comprise organic pigment core whisker-like structures14 with a conformal coating 13, typically a conducting material, andoptionally, sensitive to the vapor or gas to be sensed.

The structure of the composite medium is illustrated in FIG. 1 anddescribed hereinabove. Preferably, the encapsulating material 12 and theconformal coating may be selected for sensitivity to thegas/vapor/liquid analyte see Katritzky et al., "New Sensor-Coatings forthe Detection of Atmospheric Contamination and Water," supra. Gases,vapors or liquids typically sensed include but are not limited toacetone, methyl ethyl ketone, toluene, isopropyl alcohol, hydrogensulfide, ammonia, carbon dioxide, carbon monoxide, nitrous oxide, sulfurdioxide, organophosphorus compounds in general, dimethylmethylphosphonate, chloroethyl ethyl sulfide, xylene, benzene,1,1,1-trichloroethane, styrene, hexane, ethyl acetate,perchloro-ethylene, cyclohexane, VMP naphtha, cellosolves, chloroform,methylene chloride, Freon™ 113, ethanol, ethylene oxide, hydrogenfluoride, chlorine, hydrogen chloride, hydrogen cyanide, toluenediisocyanate, methylene di-p-phenylene isocyanate, and formaldehyde. Thepreferred sensing property of the sensor is the electrical impedance.

The sensing composite article of the present invention is a dual modesensor since the conductive conformal coating and the polymerencapsulant may each be selected for their individual response to aparticular analyte molecule of interest.

The sensing composite article is a dual sensor in a second aspect, aswell. Constructing a sensor as illustrated in FIG. 3(b), the effect ofvapor/gas molecules absorbed by the encapsulant on its dielectricproperties can be sensed by changes in the capacitance being measured.Since this impedance in the perpendicular direction is predominantlydetermined by capacitance, and is unaffected by the in-plane resistivityof the whisker surface layer, the perpendicular-capacitance and in-planeresistance values are independent.

Since the microstructure's conformal coating and the encapsulant mayindependently be chosen to have varying degrees of sensitivity to anarbitrary specific gas, vapor or liquid analyte, it is possible tocombine a variety of such individually comprised sensors into amultiplexed array, whereby the integrated response of the array as awhole to an unknown gas, vapor or liquid composition, could be used todetermine the composition of the unknown gas, vapor or liquid, therelative fractions of the components making up the later, or for asingle analyte, the absolute concentration.

EXAMPLES

Objects and advantages of this invention are further illustrated by thefollowing examples, but the particular materials and amounts thereofrecited in these examples, as well as other conditions and details,should not be construed to unduly limit this invention. Unless otherwisestated or apparent all materials used in the following examples arecommercially available.

Examples 1 and 2 illustrate the basic procedure for preparing thecomposite articles of the present invention.

EXAMPLE 1

Organic pigment C.I. Pigment Red 149,(N,N'-di(3,5-xylyl)perylene-3,4:9,10-bis(dicarboximide)) [hereinafterreferred to as perylene red], available from American Hoechst-Celenese,(Somerset, N.J.) was vacuum vapor deposited onto a stretched, 0.0125 mmthick sheet of copper coated polyimide, formed into a disc 8.3 cm indiameter. The resulting copper coated polyimide, having a 1000 Angstromthick film of perylene red, was then annealed in vacuum, heating theentire continuous perylene red film coating the polyimide, by thermalconduction through the polyimide substrate. The perylene red film washeated at approximately 280° C. over a period of 90 minutes. Aftervacuum annealing, the disc had a nanostructured layer of discrete,oriented crystalline whiskers 1 to 2 μm in length. CoCr (86%/14%) wasthen sputter coated conformally onto the whiskers, using a conventionalradio frequency (rf) glow discharge for 3 minutes at 13.7 MHz, with a 20cm diameter target, 10 cm substrate-to-target distance, 24 mTorr ofArgon (Ar), 500 watts of forward power and 1200 volts target bias.

Five milliliters (mL) of DUCO™ Cement "Household Cement" (DevconCorporation), a solution of thermoplastic resin in toluene and othersolvents, were applied to the center of the sample disc while spinningat 200 rpm. The disc was stopped when the cement flowed out to theperimeter of the sample disc. After air drying at room temperature forapproximately 5 hours, the resulting nanostructured composite easilydelaminated from the original polyimide substrate, producing a smoothsurface where the now solidified cement had interfaced with thepolyimide. The resulting dried thickness of the composite film wasapproximately 0.12 mm.

Two rectangular pieces of the composite were cut from different sectionsof the sample disc to give a sample with an area of 1.55 cm². Theend-to-end resistance of one strip was measured to be 12,060 ohms andthe second strip measured 2910 ohms. The strips were pressed togetherbetween glass microscope slides with the electrically conductingnanostructured surfaces facing outward. The sinewave output from asignal generator over the frequency range of 1 kHz to 10 MHz was appliedacross the conductive surfaces on one end of the composite strip, thatis the terminals x and w shown in FIG. 3(a). The output signal developedacross contacts y and z shown in FIG. 3(a) were monitored with anoscilloscope having a 1 megohm, 20 picofarad (pF) input impedance usinga 1× probe, or with a 10× probe having a 10 megohm, 13 pF impedance.

FIG. 5, curve (a) shows the measured output signal peak-to-peakamplitude normalized to that of the input signal. It is seen that thecomposite strips have an electronic transfer function similar to a lowpass RC network, with a fall-off of approximately 6 dB/decade.

EXAMPLE 2

A second 8 cm diameter sample disc was prepared as described in Example1, except CoCr was sputtered onto the perylene whiskers for 4 minutes atthe conditions of Example 1, followed by encapsulation with 3 ml ofDUCO™ cement. Two pairs of rectangular strips were cut from the sampleand pressed together between glass microscope slides to form twocomposite strips. The same AC signal transfer function was measured as afunction of frequency for each of these dual strips and are illustratedin FIG. 5. Referring to curve (b), the resistances of both sides of thedual composite strip (area of 2.7 cm²) were approximately 4500 ohms.Referring to curve (c), the resistances were approximately 2200 ohms andarea approximately 6.5 cm². The dual composite strip thicknesses wereapproximately 0.05 mm. The frequency "cut-off" values shift in responseto the capacitance and resistance of the strips.

EXAMPLES 3-10

The following examples illustrate a range of nanostructured compositesample types, varying with respect to the coating on the perylenewhiskers and the polymer encapsulant used, to generate a series ofequivalent RC network circuits with low-pass and high-pass cut-offfrequencies that vary over several orders of magnitude. All samples wereidentically prepared up to and including the growth of the perylenewhiskers. In each example, the sample type was identified according tothe composition listed in Table 1. In each case, Scotch™ brand aluminumfoil backed adhesive tape (3M Co., St. Paul) was applied to the polymerencapsulant side of each sample piece. Electrical contact was then madeto the ends of the sample strip on the nanostructured side, and themetal foil tape on the opposing side, to form the three terminalnetworks shown in FIG. 2. The resistance across the conducting side ofeach sample was measured with a Keithley model 617 digital electrometer.The capacitance of each sample was measured as described below. Aconventional 1.025 megohm resistor was placed in series with terminal xof a sample capacitor (FIG. 2), and a squarewave signal in the 100 Hz toseveral kHz range was applied to the 1.025 megohm resistor and terminalz (FIG. 2) across the two circuit elements. The voltage signal decayacross the sample terminals y and z was monitored with an oscilloscope(10× probe) and the RC time constant read directly from the sample'scapacitance waveform, allowing C (capacitance) to be calculated fromEquation I

    t=RC                                                       (I)

where R is the resistance in ohms, C is the capacitance in farads and tis the decay time in seconds for capacitance to discharge to 1/e of theinitial charge.

                  TABLE 1                                                         ______________________________________                                        Sample  Conformal                                                             Type    Coating        Encapsulant                                            ______________________________________                                        A       Cu             DUCO ™ Cement.sup.1                                 B       Cu             Urethane/Vinyl.sup.2                                   C       Cu             Fluorenone polyester.sup.2                             D       Ag             DUCO ™ Cement                                       E       Ag             Fluorenone polyester                                   F       Au             DUCO ™ Cement                                       G       CoCr           DUCO ™ Cement                                       H       CoCr           Fluorenone polyester                                   I       Au             Fluorenone polyester                                   ______________________________________                                         .sup.1 Devcon Corp., Danvers, MA                                              .sup.2 3M Co., St. Paul, MN                                              

Examples 3-5 illustrate the passive network response of small flexiblestrips of type D samples. A type D oriented nanostructure was made byfirst evaporating 750 Angstroms (Å) mass equivalent of Ag onto perylenewhiskers in a conventional diffusion pumped bell jar vacuum systemoperating at approximately 10⁻⁶ Torr pressure range, and thenencapsulating the nanostructure as described in Example 1. The sampleparameters are shown in Table 2. The low-pass frequency response curvesare shown in FIG. 6 and identified in Table 2. The high-pass frequencyresponse curves are shown in FIG. 7 and identified in Table 2.

Examples 6-8 illustrate the passive RC network response of smallflexible strips of type H samples. The type H oriented nanostructure wasmade by sputtering CoCr (5 minutes under the conditions of Example 1)onto perylene whiskers and encapsulating the nanostructure in fluorenonepolyester (FPE) by spin coating 7 ml of a 5% solution in cyclohexanoneat a revolution rate sufficient to just cover the entire 8 cm diametersample disc, followed by air drying for 16 hours at room temperature and4.5 hours at approximately 70° C. The sample parameters are shown inTable 2. The low-pass frequency response curves are shown in FIG. 6 andidentified in Table 2. The high-pass frequency response curves are shownin FIG. 7 and identified in Table 2.

Example 9 illustrates the passive RC network response of a smallflexible strip of type G sample. The oriented nanostructure was made bysputtering 750 Angstroms mass equivalent of CoCr onto the perylenewhiskers and encapsulating them in 5 ml of DUCO™ cement as in Example 2.The sample parameters are shown in Table 2. The low-pass frequencyresponse curves are shown in FIG. 6 and identified in Table 2. Thehigh-pass frequency response curves are shown in FIG. 7 and identifiedin Table 2.

Example 10 illustrates the passive RC network response of a smallflexible strip of type A sample. The oriented nanostructure was preparedby sputtering Cu to a mass equivalent of approximately 600 Å onto theperylene whiskers and encapsulating in DUCO™ cement. The results areshown in Table 2. The low-pass frequency response curves are shown inFIG. 6 and identified in Table 2. The high-pass frequency responsecurves are shown in FIG. 7 and identified in Table 2.

The response curves in FIG. 7 appear to be band pass frequency responsecurves rather than high-response curves. This is due to oscilloscopeinput impedance, which in combination with the sample strips' halfresistances, act as a low pass filter following the high pass circuitconfiguration shown in FIG. 2(b).

                                      TABLE 2                                     __________________________________________________________________________    Example                                                                            Sample                                                                            Area                                                                              Thickness                                                                           Resistance                                                                          Capacitance                                                                          Low-Pass Freq.                                                                          High-Pass Freq.                     No.  Type                                                                              (cm.sup.2)                                                                        (mm)  (ohms)                                                                              (pF)   Response (FIG. 6)                                                                       Response (FIG.                      __________________________________________________________________________                                              7)                                  3    D   12.1                                                                              .04     26 × 10.sup.6                                                               146    a         --                                  4    D   1.54                                                                              .04   1.86 × 10.sup.6                                                                 61.5 b         a                                   5    D   3.52                                                                              .04    .20 × 10.sup.6                                                               96     c         b                                   6    H   2.37                                                                               .025 .023 × 10.sup.6                                                               63     d         f                                   7    H   13  .02   1180-1290                                                                           425    e         d                                   8    H   3.9 .02    560  171    h         g                                   9    G   2.5 .07   1760  40     f         e                                   10   A   2.1  .064 1005  51     g         c                                   __________________________________________________________________________

EXAMPLES 11-19

In Examples 11-19, the power dissipation capability of thenanostructured composite article, used in purely a resistive mode, isdemonstrated and compared to a conventional carbon resistor and a thinmetal film coated polymer.

In Examples 11-18, thin strips of varying surface resistance, formedwith various metal/polymer combinations as described in Table 1, wereheated by passing current through the strip until the test strip failed.The strips were laid against a glass slide with the nanostructured sideagainst the glass and a temperature probe pressed against the oppositepolymer side of the strip to monitor the temperature rise as a functionof current level. The glass slide was not cooled. The plots oftemperature rise versus electrical power dissipated in the compositestrips was observed to be linear. Table 3 summarizes the results ofeight sample strips, made from five sample types, as described inTable 1. Table 3 summarizes ΔT/ΔP, the slope of the linear temperatureversus power plot, the test strip resistance, area, thickness, volume,and the maximum current density at the time of failure. The currentdensity is calculated assuming the current carrying layer of the stripis approximately 2 μm thick, which is the known thickness of thenanostructured region of the composite article.

The last entry of Table 3 identified as example 19, shows similarmeasurements from a standard 12 ohms, 1/4 Watt carbon resistor, havingcylindrical geometry. The current density of the carbon resistor iscalculated using the inner carbon volume diameter. It is seen that thenanostructured composite films can support current densities 50 to 70times larger than standard resistors of equivalent resistance andvolume, for a similar temperature rise. This is due in large part to thelarger surface area for heat dissipation. For Examples 11-19, it can beshown that the thermal conductivity of the polymer forming the bulk ofthe strip is the limiting thermal dissipation factor.

                                      TABLE 3                                     __________________________________________________________________________    Example                                                                            Sample                                                                             Area                                                                              Thickness                                                                           Volume                                                                             Resistance                                                                          ΔT/ΔP                                                                   J.sub.MAX                                No.  Type (cm.sup.2)                                                                        (mm)  (cm.sup.3)                                                                         (ohms)                                                                              (°C./Watt)                                                                   (amps/cm.sup.2)                          __________________________________________________________________________    11   F    3.4 .064  .022  7.9  44.3  1412                                                                          (ΔT = 20° C.)               12   D    3.0 .066  .020 12.7  29.0  1220                                                                          (ΔT = 16° C.)               13   D    3.2 .066  .020  16   38.5  1061                                                                          (ΔT = 19° C.)               14   B    3.6 .025  .009  33   23.0  375                                                                           (ΔT = 6.6° C.)              15   C    1.2 .051   .0063                                                                              409  23.1  438                                                                           (ΔT = 25° C.)               16   D     1.35                                                                             .038   .0051                                                                             1079  35.4  857                                                                           (ΔT = 15° C.)               17   C    2.0 .028   .0056                                                                             5250  24.1  600                                                                           (ΔT = 7.9° C.)              18   G    2.1 .097  .020 17730 27.3  225                                                                           (ΔT = 33° C.)               19   Std. NA  NA    .030 12.2  40.2   19                                           Resistor                        (ΔT = 20° C.)               __________________________________________________________________________

EXAMPLES 20-23

In Examples 20-22, sample strips similar to those described in Examples11-18 were resistively heated while heat-sinked to maximize the totalpower dissipation, and compared to Example 23, a cobalt filmsputter-deposited on 0.05 mm thick polyimide. The sample strips werepressed tightly against a water cooled copper block with a thin film ofheat transfer grease applied between the block and the polymer side ofthe nanostructured composite strips. Nextel™ (3M Co., St. Paul)insulating material was pressed against the nanostructured side of thestrip, and a 0.025 mm diameter chromel-alumel (Type K) thermocouplemeasured the temperature at the midpoint of the conducting side of thestrip through a small hole in the Nextel™ sheet. In this configuration,the surface temperature of the strip's conducting side was measured as afunction of the input power, with thermal conductivity determined by thecomposite strip's polymer and its thickness, or in the case of thecomparative Example 23, the polyimide substrate. The heat transfergrease, applied extremely thin, was observed to have a significanteffect. The bulk thermal conductivity, k, across the thickness, d, ofthe polymer strip is simply related to the temperature drop across thestrip, ΔT, the planar area of the strip, A, the electrical powerdissipated in the strip, P, as shown in Equation II. ##EQU1## Thethermal conductivity was found typically to be on the order of 2mWatts/cm² ° C., indicative of a solid, polymer material.

For Example 20, composite article was formed by evaporating gold to amass equivalent thickness of 1500 Angstroms onto an 8 cm diameter discof perylene whisker coated polyimide, and encapsulating the latter with6 ml of 4% solids FPE in cyclohexanone to form the nanostructuredsurface composite (type I) as described in Table 1. A test strip with anarea of 4.0 cm², thickness of 0.005 mm and an end-to-end resistance of10.8 ohms was placed on the Cu block assembly described above. Curve (a)in FIG. 8 shows the measured temperature difference across the stripversus power until failure of the strip occurred.

For Example 21, a surface composite of type E (see Table 1) was formedby evaporating 1500 Angstroms of Ag onto the whiskers and encapsulatingwith 10 ml of 4% FPE. A strip with an area of 4.0 cm², 0.05 mm thicknessand 2.9 ohms resistance was mounted on the Cu block assembly. Curve (b)in FIG. 8 shows the measured temperature difference across the teststrip versus the power dissipated in the strip.

For Example 22, a surface composite of type E was formed by evaporating1950 Angstroms of Ag onto the whiskers and encapsulating with 10 ml of4% FPE. A test strip with an area of 4.9 cm², thickness 0.014 mm andresistance of 2.2 ohms was placed on the Cu block assembly. Curve (c) ofFIG. 8 shows the measured temperature difference across the strip versusthe power dissipated in the strip.

For comparative Example 23, approximately 1250 Angstroms of cobalt wassputter deposited onto a 0.05 mm thick polyimide web, using theconditions of Example 1. A strip was cut with an area of 4.4 cm², and anend-to-end resistance of 2.8 ohms. Curve (d) in FIG. 8 shows thetemperature difference across the polyimide strip versus the power inputuntil failure, measured in same way as for Examples 20-22.

EXAMPLE 24

This example shows the use of the nanostructured composite film, with ametal coating and a polymer encapsulant, as a sensor capable ofreversibly responding to a saturated vapor of acetone with a rapidresponse time and a sensitivity, according to Equation III

    S=(R-R.sub.o)/R.sub.o =10%                                 (III)

The organic pigment C.I. Pigment Red 149, (N,N'-di(3,5-xylyl)perylene-3,4:9,10 bis(dicarboximide), (available from AmericanHoechst-Celenese), was vacuum vapor deposited onto a stretched, 0.05 mmthick sheet of copper coated polyimide, formed into a disc with adiameter of 8.3 cm. The disc was vacuum annealed to form ananostructured layer of discrete, oriented crystalline whiskersapproximately 1.5 micrometers tall, as described in Example 1. CoCr(86%/14%) was then sputter coated conformally onto the whiskers, using aconventional rf glow discharge at 13.7 MHz for 8 minutes with 20 cmdiameter targets, 10 cm substrate-to-target distance, 24 mTorr of Ar,500 Watts forward power, 1200 volts target bias and water cooling of thetarget and substrate.

Three milliliters of uncured photopolymer, (cyclohexyl methacrylate,hexamethylene diisocyanate trimethylolpropane 5 (CHMA, HMDI-TA5)), asprepared in U.S. Pat. No. 4,785,064 was applied to the center of thepolyimide disc and hand tilted to cause the solution to uniformly flowover and encapsulate the CoCr coated perylene whiskers. The photopolymerwas then cured by exposing it to the appropriate UV lamps, under N₂, forone-half hour.

The resulting nanostructured composite easily delaminated from theoriginal polyimide substrate, (FIG. 1) producing a smooth, reflectivesurface where the now solidified polymer encapsulant had interfaced withthe polyimide. An irregularly shaped piece of the brittle composite,approximately 5 cm long, 1.25 cm wide at the center and 0.5 cm wide ateach end, was broken from the original disc. Electrical leads wereattached to the ends by crimping on tinned solder lugs and coating themwith conductive paint. The total resistance of the sample piece asdescribed, was 843 ohms.

With leads from a Keithley model 616 electrometer attached to measurethe resistance, and the latter driving a time based chart recorder, thesample was placed inside a covered 400 ml polyethylene beaker. With onlyair in the beaker, the resistance remained constant at 840 ohms forapproximately 40 minutes. Acetone was then added to the covered beakerto a depth of 3 mm, so as to expose the sensor to a saturated vapor. Theresistance (R) began to increase and rose to 855 ohms over a two minuteinterval. For approximately 15 minutes, the R remained at 855 ohms andthen R increased sharply again to approximately 875 ohms over a periodof 30 seconds and remained constant for 12 minutes. R then jumped to 900ohms in a period of two minutes, thereafter remaining in the range of900 to 880 ohms for 70 minutes. At this point, the sensor assembly wasremoved from the beaker and laid on the laboratory bench, whereupon Rbegan dropping within seconds, reaching 790 ohms in 7 minutes andstaying constant for 12 minutes until put back into the acetone vapor. Rimmediately began increasing again, reaching 900 ohms in 9 minutes whereit remained constant.

In summary, this example of a nanostructured composite sensor with CoCrconformal coating and CHMA, HMDI-TA5 encapsulant has demonstrated thecapability to rapidly and reversibly sense a room temperature saturatedvapor of acetone with a sensitivity of approximately 10%.

The following example classes demonstrate the utility of thenanostructured composite film as gas/vapor sensors for H₂ S, Hg vapor,H₂ O and organic vapors of methyl ethyl ketone (MEK), acetone, toluene,isopropyl and ethyl alcohol, and as a liquid analyte sensor for aqueousBr. In all cases basic whisker structured perylene films deposited oncopper coated polyimide sheets such as described in Example 24 were usedas the starting point, and various combinations of metal conformalcoatings and polymer encapsulants used to form thirteen different typesof nanostructured composite films. Table 4 lists these different samplesaccording to metal coating and polymer encapsulant as types A-M, whichare referenced for brevity in the following Examples.

Photopolymer A was prepared as described in U.S. Pat. No. 4,510,593,Examples 6 and 2 and is incorporated herein by reference.

Photopolymer B is a radiation-curable composition prepared as describedin U.S. Pat. No. 4,986,496, Example 4 and is incorporated herein byreference.

    ______________________________________                                        Components              Parts                                                 ______________________________________                                        urethane acrylate oligomer                                                                            68                                                    (XP51-85 ™, Cargile, Inc.)                                                 tetraethylene glycol diacrylate                                                                       19                                                    (SR-268 ™, Sartomer, Co.)                                                  diethoxyacetonphenone   5                                                     (DEAP ™, Upjohn Co.)                                                       fluorochemical surfactant                                                                             2.5                                                   (FC-431 ™, 3M Co.)                                                         n-vinyl pyrrolidone     5                                                     (GAF, Inc.)                                                                   UV light stabilizer     0.5                                                   (TINUVIN 770 ™, Ciba Geigy, Inc.)                                          ______________________________________                                    

                  TABLE 4                                                         ______________________________________                                        Sample  Conformal                                                             Type    Coating        Encapsulant                                            ______________________________________                                        A       Ag             Photopolymer A.sup.1                                   B       Ag             DUCO ™ Cement.sup.2                                 C       Ag             Photopolymer B.sup.3                                   D       Ag             Vinol polyvinyl alcohol                                E       Cu             Fluorenone polyester.sup.4                             F       Au             UV optical adhesive.sup.5                              G       Au             DUCO ™ Cement                                       H       CoCr           Photopolymer                                                                  CHMA, HMDI-TA5.sup.6                                   I       CoCr           Photopolymer A                                         J       CoCr           Fluorenone polyester                                   K       Fe             DUCO ™ Cement                                       L       Cu             DUCO ™ Cement                                       M       CoCr           DUCO ™ Cement                                       ______________________________________                                         .sup.1 U.S. Pat. No. 4,510,593                                                .sup.2 Devcon Corp., Danvers, MA                                              .sup.3 U.S. Pat. No. 4,986,496                                                .sup.4 3M Co., St. Paul, MN                                                   .sup.5 Norland Products, Inc., New Brunswick, NJ                              .sup.6 U.S. Pat. No. 4,785,064 (cyclohexyl methacrylate, hexamethylene        diisocyanate trimethylolpropane 5)                                       

EXAMPLES 25-33

These examples demonstrate the utility of samples of types A, B, C, D,and E as irreversible sensors or dosimeters for H₂ S gas underconditions of high humidity, and illustrate the dependence of sensorsensitivity on the initial resistivity for quantitative analyses.

Example 25 illustrates that type B samples produce a significantresponse to H₂ S/N₂ concentrations as low as 30 ppm in times as short as30 seconds under conditions of 50% relative humidity (R.H.) and 30 1/minflow rates.

A strip of type B sample, made by evaporating 900 Angstroms massequivalent of Ag and encapsulating to a thickness of 0.045 mm, was cut 6mm wide and 4.5 cm long. Electrical contact to the strip was made bysimply clipping smooth-jawed miniature alligator clips to the ends ofthe strip. The initial resistance was 870 ohms. The strip was supportedwithin a sealed 9 oz. glass jar and the resistance continuouslymonitored while gas mixtures of known composition and flow rate wereadmitted and allowed to vent through tubes penetrating the jar cover.Two sources of gases were mixed in a preliminary 9 oz. jar that suppliedthe final mixed gas to the sample containing jar. The first gas sourcewas wet N₂, produced by flowing N₂ over a humidistat (General Eastern)controlled water vapor bath through a glass flow meter tube (LabCrestNo. 450-688).

The second gas source was either pure N₂ or 108 ppm H₂ S/N₂ (UnionCarbide Industrial Gases Inc.) supplied to the mixing jar via a flowmeter (Ace Glass Inc., tube #35). While flowing only humidified N₂ intothe sample jar, the resistance remained constant at 869 ohms over aperiod of 35 minutes during which time the relative humidity wasinceased from 50% to 78% in the first gas source, flowing at 16 1/min.,and the second gas source of dry N₂ flowed at approximately 14 1/min.,to produce a total flow of approximately 30 1/min. of humidified N₂ atapproximately 25% to 39% RH. This demonstrates that a type B sensor isunaffected by water vapor, such as would exist in the vicinity of humanbreath. The valving of the second gas source was quickly switched toadmit the 108 ppm H₂ S at approximately 5 1/min. in place of the dry N₂,and immediately the resistance began rapidly dropping at a rateexceeding 100 ohms/min. as shown in FIG. 9 (a), finally approaching astable and nonreversible resistance of 490 ohms, a decrease of 44%. Thisrepresents an average relative resistance change of 11%/min. over thefirst two minutes. The relative flow rates and mixture values imply thatthe rapid and large response of the strip's resistance was produced byapproximately 30 ppm H₂ S/N₂ at approximately 40% RH.

For Example 26, a second strip of type B sample, 4.5 cm ×5 mm, having alower resistivity than Example 25, was mounted in the same testapparatus as Example 25. The initial resistance of the strip wasconstant at 130.7 ohms while exposed to the wet N₂ gas mixture flowinginto the mixing jar at approximately 23 1/min. and 55% RH. Uponswitching to a mixture containing 35 ppm H₂ S gas, the resistance begandropping within seconds, reaching approximately 105 ohms in 2 minutesand eventually 82.2 ohms after 20 minutes. This represents an averagerelative change of resistance of approximately 10%/min. over the firsttwo minutes, similar to Example 25, despite the difference in initialresistivity. It should be noted that the very stable resistance tonearly 1 part per thousand implies that even just a 25 ohms change from130 ohms is still a signal to noise ratio of 20%/0.1% or 200/1.

For Example 27, a third strip of type B sample used in Examples 25 and26 had an initial resistance of 3900 ohms. Unlike the previous examples,some sensitivity to water vapor was noted. The strip was mounted in thesame test apparatus, but a simpler gas admission system was used inwhich either dry N₂ or the 108 ppm H₂ S//N₂ gas mixture could beadmitted directly to the jar and vented through a second tube in the jarcover. Two ml of distilled water was added to the bottom of the testapparatus. The flow rates were not quantified, but produced a fastbubble out the 3 mm diameter (O.D.) vent tube when its outer end wasplaced in water. Upon switching from the pure N₂ to the H₂ S/N₂ gas, thesample resistance increased briefly to 4150 ohms over 30 seconds, thenplummeted to 1000 ohms in 90 seconds, a sensitivity of 50%/min, andreached 260 ohms after 6 minutes. In comparison to the previousexamples, this example indicates that the nanostructured compositesensitivity to H₂ S may be correlated to the initial resistivity.

For Example 28, a similar strip of type B sample, with 1000 Angstromsmass equivalent Ag, and a very low initial resistance of 15.4 ohms, wasexposed to the same gas flow conditions as used in Examples 25 and 26.No response to the H₂ S gas was noted after switching from wet N₂. Thiscomparative example to Examples 25 to 27 indicates that too low aninitial resistivity is not desirable. Sensitivity may be correlated toinitial resistivity, probably because a different conduction mechanismmay be dominating the current flow which is less sensitive to initialsmall degrees of reaction with the H₂ S.

For Example 29, a strip of type D sample, made by annealing the perylenewhiskers at 240° C. for 80 minutes, vacuum evaporating 1000 Angstromsmass equivalent of Ag onto the whiskers, and solution coating with a 5%solution of vinol-polyvinyl alcohol in water with 0.1% Triton X-100,(Rohm & Haas, Philadelphia, Pa.) was cut 5 cm long and 6 mm wide. Thestrip's initial resistance under flowing dry N₂ in the simpler gas flowarrangement of Example 27 was 34 ohms. Switching to the 108 ppm H₂ S/N₂gas source, in the absence of any water vapor in the apparatus producedno change in resistance. Three milliliters of water were added to theapparatus and the gas flow sequence repeated. With water vapor present,the resistance began dropping 90 seconds after switching to the H₂ S andexhibited a relative resistance drop of 10% over a period ofapproximately 11 minutes. This degree of change is also qualitativelyconsistent for such a low initial resistivity and the observations ofthe previous examples. It also indicates with this polymer encapsulantthe need for finite relative humidity.

For Example 30, a strip of type C sample, made by annealing the peryleneat 280° C. for 90 minutes, vacuum evaporating 1035 Angstroms massequivalent of Ag and spin coating 3 ml of photopolymer B onto an 8 cmdiameter disc and UV curing, was cut 3.7 cm long and 6 mm wide. Thestrip was exposed to N₂ and a 108 ppm H₂ S/N₂ gas mixture in the simplergas flow arrangement of Example 27. Distilled water was present in thebottom of the apparatus. The initial resistance of the strip was 1.36Kohms and remained constant in a pure N₂ flow. Switching to the 108 ppmH₂ S/N₂ flow, the resistance began dropping after 1.0 minute, anddecreased logarithmically as shown in FIG. 10.

For Example 31, a strip of type B sample with an initial resistance of898 ohms was placed in the simpler gas flow arrangement of Example 27.Distilled water was present in the bottom of the apparatus. With onlypure N₂ flowing into the apparatus, adequate to produce a fast bubblefrom the outlet tube, the resistance was constant. Upon switching to the108 ppm H₂ S/N₂ flow, the resistance versus time curve broke at t=12seconds after the start of the H₂ S flow, as shown in FIG. 9 (b),dropping at an initial rate of 36%/min, reaching less than 400 ohmswithin two minutes.

Example 32 shows that the resistance can also increase due to H₂ Sexposure with a different metal coating on the whiskers and polymerencapsulant.

An example of type E was prepared by annealing the perylene whiskers for160 minutes and sputter coating Cu for two minutes under the same rf andAr pressure as cited in Example 24. It was then encapsulated influorenone polyester (FPE) by spin coating 5 ml of a 5% solution of FPEin cyclohexanone at a rate of 170 rpm onto an 8 cm diameter disc sample.After air drying, the FPE encapsulated nanostructured composite wascleanly delaminated from the original copper coated polyimide substrate.A piece with an initial resistance of 104 ohms was cut and mounted inthe simpler gas flow arrangement described in Example 27. There was nowater vapor present. Upon switching from pure N₂ to the 108 ppm H₂ S/N₂flow the resistance began increasing very slowly as shown in FIG. 11.After several hours the resistance had increased to only 113.7 ohms, atwhich point the flow was stopped and the remnant vapors left in thesealed apparatus. As indicated in FIG. 11 the resistance began changingmuch more rapidly during the static conditions, until the pure N₂ flowwas restarted.

In contrast to Examples 25 to 31, Example 33 illustrates that theresistance can increase upon H₂ S exposure even if Ag is used to coatthe whiskers but a different encapsulant is used.

A type A sample was prepared by vapor coating a whiskered perylenesample with approximately 800 Angstroms of Ag, spin coating 5 ml of theORP photopolymer at 450 rpm onto an 8 cm diameter sample disc, and UVcuring. A piece with an initial resistance of 40.8 ohms was mounted inthe simpler gas flow arrangement of Example 27. Distilled water waspresent in the bottom of the apparatus. With pure N₂ flowing, theresistance was constant to within 0.1 ohm. Upon switching to the 108ppm/N₂ flow, the resistance began changing within 30 seconds, dippedbriefly to 40.0 ohms over 2 minutes, and then began increasingmonotonically, ultimately reaching 54.5 ohms after approximately 90minutes. As in Example 32, it was observed that under static conditions,the rate of resistance change was faster than with positive flow. Thiseffect is interpreted as due to nonequilibrium gas mixing in the simplesingle jar flow arrangement described in Example 27, and lowering of therelative humidity in the jar when the dry gas mixture is admitted.

In summary, Examples 25 to 33 show that the polymer encapsulant andmetal coating both contribute to the response of the sensor to H₂ S,even causing the resistance to change in opposite directions fordifferent combinations of metal and encapsulant, providing evidence thatthe conduction mechanisms and gas sensitivity involve both constituentsof the nanostructure, unlike the prior art. These examples also showthat a particularly useful combination for a fast, sensitive H₂ Ssensor, which is not affected by humid air alone is Ag and DUCO™ cement.

Specifically for this combination and 108 ppm H₂ S/N₂, Examples 27, 28and 31 illustrate a logarithmic dependence of the initial rate ofrelative resistance change, 1/R_(o) (dR_(o) /dt), (or dS_(o) /dt in%/min), on initial resistance, R_(o), as shown in FIG. 17. This resultis potentially very important since it indicates a simple means toquantify the analyte concentration with these sensors. It is logical toassume that the slope of the straight line in FIG. 17 will vary with therelative concentration of H₂ S, and in fact this is supported byExamples 25 and 26 where effective concentrations of 30 and 35 ppmrespectively produced initial sensitivity rates of approximately 10%/minfor initial resistances of 870 and 131 ohms respectively. These datapoints are shown in FIG. 17 with dashed lines to indicate thehypothetical slope at those concentrations. (Note that all samples aretype B samples). Given then two or more sensors, of different initialresistance, responding to the same gas concentration, it is onlynecessary to measure the initial rate of resistance change for eachsensor, over one to two minutes, calculate the slope of a plot of theinitial resistance rate of change versus resistance as indicated in FIG.17, and compare to a calibration table to determine the analyteconcentration. All of this could be done via integrated circuitry, and amultiplexed array of several of these sensors of regularly varyingresistance could perhaps give good accuracy as well.

EXAMPLES 34 and 35

Examples 34 and 35 show that using gold as the conformal coating on thewhiskers produces a mercury vapor sensor with a reaction mechanismdominated by solid state diffusion.

For Example 34, a type F sample was prepared by vapor coating 1500Angstroms mass equivalent of gold onto a whisker coated substrate disc,spin coating with NOA 81 optical adhesive (Norland Products, Inc.) atapproximately 250 rpm producing a UV cured film approximately 0.3 mmthick. The nanostructured composite was cleanly delaminated from thepolyimide substrate. Strips of the composite, approximately 5 mm ×35 mm,were cut and individually mounted inside a test apparatus comprising asealed 9 oz. glass jar having electrical leads penetrating the jarcover. The initial resistances were in the range of 120 to 600 ohms. TheHg vapor was generated by adding a few milliliters of pure Hg to theapparatus and placing the apparatus assembly into an air oven atcontrolled temperatures. After a resistance versus time run was completeat one temperature, a new sample strip was used to obtain another run ata new temperature, and hence vapor pressure. The Hg vapor pressure,hence the concentration was taken as the equilibrium vapor pressure ofHg at the given temperature. Four sample strips were exposed in thismanner to Hg vapor between room temperature and 91° C. The sensitivity,defined by Equation III is plotted in FIG. 12 and demonstrates a strongtemperature dependence and approximately a square root of timedependence, an indication that solid state diffusion is the ratelimiting step.

Assuming a model for the gold coated whisker composite as a distributionof gold "posts" which are being converted to a AuHg alloy by solid statediffusion of Hg through the alloy to the alloy/Au interface, which ispropagating down the length of the "post" as t^(1/2), the sensitivitycan be expressed in terms of the resistivities of the Au and alloy, anda diffusion coefficient of Hg through the alloy. FIG. 18 shows a plot ofsensitivity versus reciprocal temperature from which the temperaturedependence of the diffusion coefficient can be extracted as shown.

For Example 35, a type G sample was prepared by vapor coating a massequivalent of 2500 Angstroms of Au onto a whisker coated substrate,followed by encapsulation in DUCO™ cement by spin coating 4 ml of theadhesive at 440 rpm onto an 8 cm diameter polyimide disc and allowing itto air dry. As in Example 34, strips were cut from the delaminatedcomposite and exposed to mercury vapor at various temperatures. Theresistances of the strips were initially in the range of 5 to 20 ohms.For each new temperature, the sample was first monitored in a Hg freeapparatus at the designated temperature, establishing that at anytemperature the resistance was constant in air. When switched to the Hgcontaining apparatus, the resistance increased as shown by thesensitivity plot in FIG. 13, wherein a significantly larger response isrecorded than with the type F sample in Example 34.

Examples 34 and 35 show that the sensitivity of the sensor for Hg, usingAu as the reactive metal, is strongly dependent on the initial sampleresistance, assuming the type of polymer is not as important in thiscase, and that a reaction mechanism can be extracted suggesting solidstate diffusion and amalgamation. This implies that even more sensitiveHg vapor sensors could be created with different metal coatings, such asAl, although the Au may be beneficial where no reaction to water vaporor high temperature is desired.

EXAMPLE 36

Example 36 shows that using copper as the conformal coating on thewhiskers produces an irreversible sensitive indicator of total watervapor exposure.

For Example 36, a type E sample was prepared as described in Example 32.Several strips were cut, 2-3 cm long and 6-8 mm wide, having initialresistances in the 50 to a few hundred ohms range. The strips, attachedto the leads of an electrometer, were suspended directly over warmedwater contained in an insulated Dewar. The resistance change versus timewas then recorded for different average water temperatures, the lattergenerally remaining constant to within 1 degree during the exposure.FIG. 14 shows a summary plot of the sensitivity, S=(R-R_(o))/R_(o),versus time for seven strips at different water temperatures, and hencerelative humidities. As indicated, the resistance change can vary overseveral orders of magnitude as the Cu coating on the whiskers isoxidized. There appear to be at least two regimes of behavior orkinetics, and the mid-range can be modelled assuming an Arrheniusrelationship between S and exposure time, S=S_(o) exp(mt). Using theslope at the inflection points of the log(S) vs. time curves to get m asa function of temperature, the latter is plotted vs. reciprocaltemperature in FIG. 16, from which an apparent heat of enthalpy of only11 kcal/mole results. This is considerably lower than reported heats offormation of most metal oxides, in the 30-200 kcal/mole range, with Cuapproximately 60 kcal/mole. This high reactivity is probably aconsequence of the high surface area of the whiskers and small size,which leads to faster response and greater sensitivity than solid thinfilm based sensors would exhibit.

EXAMPLE 37

Example 37 demonstrates the potential for selecting the polymerencapsulant to specifically sense organic solvent vapors. The gasconcentrations were not varied or controlled in these experiments,rather the room temperature vapor pressures were used to simplydemonstrate for different metal/polymer combinations that the responsecan be reversible and that the sensitivity is very dependent on thepermeability of the gas or vapor into the encapsulant.

In Example 37, nanostructured film samples were made using fourdifferent combinations of metal coating and polymer encapsulant, inaddition to that of Example 24. Strips similar to those described inprevious Examples were cut from sample discs and electrical contact madeto the ends by various means, including crimped indium foil andconductive silver paint. The resistance of each strip was then recordedagainst time, first with each strip suspended within a dry polyethylenebeaker, in air, and then continuing after solvent was added to thebottom of the beaker.

Table 5 summarizes the observed average rate of resistance change forvarious solvents and indicates whether the response was reversible, whentested over one or two cycles. The total change, expressed assensitivity S is also given where appropriate. The rate of sensitivitychange increases with initial resistivity, since all pieces were aboutthe same size and shape. Also, the affinity of the polymer for thesolvent is presumably the primary reason for the large disparity ofresponse of a single sensor type to various solvents.

                  TABLE 5                                                         ______________________________________                                        Sample                                                                              Solvent   R.sub.0 ΔR/Δt                                     Type  Vapor     (ohms)  (ohms/min)                                                                             S (%) Reversible                             ______________________________________                                        E     MEK        60     0.5       ˜ 25                                                                         yes                                    E     Acetone   1000    6        --    --                                     E     Acetone   8080    670      --    --                                     E     Toluene   105     ˜ 0                                                                              --    --                                     E     Toluene   900     1        --    yes                                    E     Isopropyl   86.5  0.025    --    --                                           Alcohol                                                                 K     Acetone   278     18       --    --                                     K     MEK       487     200-500  750   no                                     K     MEK       466     200-270  900   no                                     J     Acetone   460     22        38   yes                                    J     MEK       600     10        72   yes                                    J     Toluene   1460    40        27   yes                                    J     C.sub.2 F.sub.3 Cl.sub.2                                                                1350    4         6    yes                                    I     Acetone   377     30       300   yes                                    ______________________________________                                    

The resistance changes, which in all cases were increases, occur mostprobably due to both polymer swelling and the attendant increase ininter-whisker spacing, and changes in the intrinsic electronic transportproperties of the interwhisker polymer material after sorbing solventmolecules. Similar resistance changes in three dimensional dispersionsof carbon black particles in polymers is well known and often describedin terms of percolation theory and the Hildebrand solubility parameterof the polymer and solvent. It is also conceivable that thepolymer/metal interface of the coated whiskers is a controlling factorin charge injection or tunneling as well. The potential to tailor thepolymer and metal combination for desired responses from specific gasesor vapors would appear feasible. The results suggest, as with H₂ S, thatthe rate of sensitivity change may be correlatable with vaporconcentration.

EXAMPLE 38

Example 38 demonstrates the use of the composite medium as a liquidanalyte sensor.

A piece of type B sample, 6 mm ×40 mm, was held in the form of asemicircle by the electrical leads from an electrometer and immersed ina 180 ml volume of distilled water, contained in a 250 ml glass jar, sothat the electrical clip leads were just above the water surface. Theinitial air resistance, 149.3 ohms, did not change when immersed in thewater, until after several minutes, at which time the resistance beganto slowly decrease at a constant rate of 0.26 ohms/min. over a period of45 minutes. At this point, 1 milliliter of an equilibrium solution ofBr₂ in distilled water was injected by syringe into the center of the180 ml sample volume and briefly stirred, giving a 68 ppm aqueous Brsolution. The resistance remained stable for approximately 2 minutes,then began rapidly decreasing at a rate of approximately 4 ohms/min.,finally equilibrating after fifteen minutes for a total sensitivitychange of 26%.

EXAMPLE 39

Example 39 demonstrates that the capacitance measured perpendicular tothe film plane can be used as the sensor property.

Small area pieces, on the order of 2 cm², were cut from B, E, L, and Mtype samples. Aluminum foil backed adhesive tape was applied to thepolymer encapsulant side of each piece so as to form a simple capacitorwith the nanostructured surface of the composite forming one conductiveside of the capacitor and the aluminum foil the other. The thickness ofthe adhesive layer on the tape was much thinner than the composite film,the latter being in the 0.05 to 0.125 mm thickness range. Electricalcontact to the formed capacitor was made by smooth jawed alligatorclips. To measure the capacitance, a 1.025 megohm resistor was placed inseries with the sample capacitor, a squarewave signal in the 100 Hz toseveral kHz range was applied across the two circuit elements, and thevoltage signal across the sample capacitor was monitored with anoscilloscope. The RC time constant was read directly from the samplecapacitor waveform, and the sample's capacitance calculated fromEquation I. The sample test assembly was then placed in a coveredbeaker, directly over acetone liquid in the bottom, to expose the samplecapacitor to a nominally saturated vapor. The RC time constant was thenperiodically read off the oscilloscope, the total circuit capacitancecalculated and the scope probe capacitance subtracted to give the samplecapacitance. FIG. 15 shows the variation of sample capacitance withelapsed time for the four sample types, beginning at t=0 when thesamples were placed in the vapor.

By combining the capacitance property of the Example with thedemonstrated surface resistance properties of Example 37, it is clearthat the sensor media could be made into a resonant, tuned circuitelement with a fast frequency shift type response.

Various modifications and alterations of this invention will becomeapparent to those skilled in the art without departing from the scopeand spirit of this invention, and it should be understood that thisinvention is not to be unduly limited to the illustrative embodimentsset forth herein.

I claim:
 1. A composite article with an electrically conductive surface,comprising an encapsulating layer having a regular or random array ofdiscrete microstructures, each microstructure comprising an organiccompound wherein the molecule thereof is planar and comprises claims orrings over which π-electron density is extensively delocalized and anoptional conformal coating enveloping said whisker-like structure,wherein said array of discrete microstructures is encapsulated in saidencapsulating layer, such that one end of each microstructure isembedded within said encapsulating layer and the other end of eachmicrostructure is exposed and coincident with the surface of saidencapsulating layer.
 2. The composite article according to claim 1,wherein said organic material is selected from the group consisting ofpolynuclear aromatic hydrocarbons and heterocyclic aromatic compounds.3. The composite article according to claim 2, wherein said organicmaterial is selected from the group consisting of perylenes,phthalocyanines, and porphyrins.
 4. The composite article according toclaim 3, wherein said organic material isN,N'-di(3,5-xylyl)perylene-3,4:9,10 bis(dicarboximide).
 5. The compositearticle according to claim 1, wherein said conformal coating is aconducting material selected from a group consisting of metals,inorganic compounds, and conducting polymers.
 6. The composite articleaccording to claim 5, wherein said metal is selected from the groupconsisting of aluminum, cobalt, cobalt chromium, nickel, nickelchromium, platinum, silver, gold, iron, copper and other transitionmetals.
 7. The composite article according to claim 1, wherein saidencapsulating material is organic or an inorganic material.
 8. Thecomposite article according to claim 7, wherein said organic material isselected from the group consisting of organic pigments, thermoplasticpolymers and co-polymers derived from olefins and other vinyl monomers,condensation and addition polymers, and natural polymers and theirderivatives.
 9. The composite article according to claim 7, wherein saidinorganic material is selected from the group consisting of gels, sols,or semiconductor, or metal oxides.
 10. The composite article accordingto claim 1, wherein capacitive impedance is measured perpendicular tothe surface plane of said article and resistive impedence is measuredparallel to the surface plane of said article.
 11. A sensor comprisingthe composite article in accordance with claim 1, wherein said conformalcoating, said encapsulating material and said whisker-like structuresare independently selected for their responsiveness to gas, vapor, orliquid analytes.
 12. A flexible thin film according to claim
 11. 13. Thecomposite article according to claim 4, wherein said organic pigmentsare selected from the group consisting of perylene red, phthalocyanine,and porphyrins.
 14. The composite article according to claim 8, whereinsaid thermoplastic polymers and co-polymers are selected from the groupconsisting of polyesters, polyimides, polyamides, polyethers,polyurethanes, and polyureas.